1. Field of the Invention
The present invention relates to a driving force transmitting device applied to conveyance means such as automobiles.
2. Description of the Related Art
An automobile is sometimes equipped with an electromagnetically operational driving force transmitting device by which torque transmission is controllably switched on and off. This device, however, requires a very powerful solenoid because great torque sufficient to drive the automobile acts on the clutch and engagement of the clutch must be achieved against this great torque.
To reduce required capacity of the solenoid, a pilot clutch may be used in combination with a main clutch. A solenoid with a relatively small capacity operates the pilot clutch to convert a part of torque into axial force that is powerful enough to engage the main clutch against the torque.
Related arts are disclosed in Patent Publications of Unexamined Japanese Patent Applications that are laid open under serial Nos. JP 2002-39216, JP 2000-240684 and JP 2008-261434.